The effect of pergolide mesylate, a new potent longacting dopamine agonist, was investigated in eight patients with active acromegaly, none of whom had received pituitary irradiation. Patients were assessed at different dose levels for clinical response and biochemical response by measuring mean fasting growth hormone (GH), mean ambulant GH and glucose-induced GH response. Six patients had definite clinical improvement. No major side effects were noted. All patients showed a statistically significant reduction in GH, although in two reduction was not below 75% of pretreatment levels. On 500 micrograms pergolide daily the overall response for the group of eight patients was a reduction of fasting serum GH, mean ambulant GH and of GH response to glucose to 55% of pretreatment values. When the daily dose was increased to 1000 micrograms there was a further significant reduction in GH in some patients; on this dose the response for the group was a decrease in fasting serum GH and mean ambulant GH to 40% of pretreatment values. In three patients GH concentrations of 5 mU/l or less were achieved. In two patients the daily dose was increased to 1500 micrograms but no extra benefit was obtained. We conclude that pergolide given once daily has a beneficial effect in acromegaly, with significant reduction in serum GH; it promises to be a useful therapeutic agent.